A seat belt system for restraining an occupant of a vehicle ordinarily includes seat belt webbing, a seat belt buckle, and a seat belt retractor. A locking tongue is connected to the webbing and is releasably lockable in the buckle when the webbing is extended around the vehicle occupant. The retractor includes a spool upon which the webbing is wound. The spool rotates in an unwinding direction when the vehicle occupant extracts the webbing from the retractor and moves the webbing toward the extended position in which the tongue is locked in the buckle. When the tongue is subsequently unlocked and released from the buckle, a rewind spring in the retractor rotates the spool in a winding direction to retract the webbing into the retractor.
When the vehicle experiences a collision, the vehicle decelerates and the vehicle occupant wearing the seat belt webbing moves forward against the webbing. The force which is then applied to the webbing urges the webbing to move outward from the retractor. Therefore, a seat belt retractor typically includes an emergency locking mechanism which blocks unwinding rotation of the spool in response to sudden vehicle deceleration and/or sudden extracting movement of the webbing. The emergency locking mechanism thus prevents the webbing from moving outward from the retractor when the vehicle experiences a collision. The webbing then restrains forward movement of the vehicle occupant. In a high energy collision, the tension in webbing may cause excessive loads to be imparted to the occupant. This may result from factors such as the weight of the occupant, the position of the occupant, and/or the energy of the collision.